This invention relates to generating and detecting radiation, especially optical radiation with wavelength lying in the far infrared, infrared, visible, or intermediate regions of the spectrum.
Coherent optical radiation has been generated using lasers and bulk non-linear media. Prior to my work, visible and near infrared radiation had not been detected in direct response to an induced alternating current flowing at the frequency of the applied radiation, other types of detectors have had deficient characteristics, especially when operating without special provision for cooling. However to a certain limited extent work involving a cat whisker as a detector at relatively low frequencies has been previously published, see Hocker and Javan, Frequency Mixing In Infrared and Far Infrared Using Metal to Metal point contact diode; Applied Physics Letters, Vol. 12; No. 12, June 15, 1968 pp 401,402.
The present invention features generating coherent optical radiation by providing an antenna connected to a metal-to-metal diode junction with non-linear characteristics, (as by maintaining the pointed end of a metal cat whisker in contact with a metal base or by use of solid overlapped metal deposits) and coupling to the diode junction electromagnetic energy sufficient to interact with the junction to cause generation of current through the junction, flow of the current in the antenna and emission of radiation from the antenna, the current and radiation being of optical frequency which is related to the frequency of the input and absent from the input.
Preferred embodiments of the invention feature collecting the radiation emitted from the antenna for direction to a station for utilization, using a modulated microwave input to cause the emitted radiation to be correspondingly modulated, mixing such microwave input with an input of optical frequency to produce the optical radiation, applying a d.c. biasing voltage across the junction to shift the operating point on the current-voltage characteristic into a region for optimum performance when the emitted radiation is produced by even order mixing, making the pointed end of the whisker with a radius of curvature less than 50 nm., making the diameter of the whisker smaller than the wavelength of the emitted radiation, making integrated solid diodes, in particular with microscopic overlapping line structures as described below singly and in phased arrays, promoting coupling and using the diode construction in combination with an absorbtion cell and a detector in spectroscopic analysis, using a feedback loop to control the emission of the diode in a frequency source, and using a demodulating diode to extract a signal from an optical beam in a communication system.
The invention achieves a high degree of control of the frequency of emitted optical radiation and particularly frequency stabilization of optical radiation for use, for example, in a frequency standard; controlled sweeping of the frequency of optical radiation for use, for example, in spectroscopic analysis; and modulation of an optical carrier beam over a wide band, including high frequencies, for use, for example, in a communication system.
The invention also features, both for use in generation of radiation, but also with application to detection, an integrated solid construction comprising successive metal deposits having a limited, i.e. microscopic, common area at which the deposits are separated by a thin dielectric layer, preferably sufficiently thin to allow electron tunneling and preferably the diode or a succession of them being achieved by microscopic line structures overlapping at their tips, the line structure serving as antennae for receiving incident optical radiation to produce an electrical effect such as mixing, production of a voltage across the junction, or in the case of generation, producing an alternating electrical current which, in proceeding to the antenna, causes radiation at the new synthesized frequency.
Preferred embodiments according to this aspect of the invention feature a plurality of such antennas and junctions deposited in an array on a substrate; series connection of array antennas and junctions to provide a composite antenna structure with successive connections effective as rectifying junctions alternating with non-responsive connections, the non-responsive connections being made so by use of an ohmic connection, by having an excessive area, by being placed at a node of a standing wave; or cancellation of rectifying voltages in successive junctions is avoided by use of a series d.c. bias; enhancement of radiative coupling by making each of the deposited antennas a length n .lambda./4 where n is a chosen integer and .lambda. is the wave length of a frequency of interest to enable effective coupling, by making each of the antennas a length onger than the 1/e attenuation length characteristic of the antenna relative to the frequency of interest, by making each antenna non-resonant and much shorter in length than the wave length of the radiation of interest, by positioning antenna-forming deposits into a phased array relative to an optical frequency of interest; or by arraying the antennas in a Franklin-Marconi geometry with co-linear antenna sections and re-entrant sections, the colinear sections constructed to relate to optical radiation in phase and the two legs of each re-entrant section constructed to cancel the effects of each other and thereby not relate to radiation out of phase with said in phase radiation; the positioning of diode junctions at the extreme excursion of re-entrant sections, or in the middle of colinear antenna sections.
Embodiments also feature: linear, planar, and 3-dimensional arrays of antenna-junction devices deposited on a substrate and arranged to produce cooperative directional radiation and use of a transparent substrate and a reflective coating at a predetermined depth or of integrated optics to enhance coupling.
The integrated diodes are employed according to the invention as detectors either for specific frequencies or over a range of optical frequencies, e.g. as room-temperature broad band detectors or power converters. A dense array of the devices is featured to resolve an image for scanning purposes.
Further features and advantages of the invention will appear from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.